<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>flex--真等高</title>
  <style>
    .box {
      display: flex;
    }
    .left {
      flex: 0 0 100px;
      background: lightgoldenrodyellow;
      min-height: 600px;
    }
    .right {
      flex: auto;
      background: darkcyan;
    }
  </style>
</head>
<body>
  <div class="box">
    <div class="left">
      这是left
    </div>
    <div class="right">
      <p>这是right</p>
      <p>这是right</p>
      <p>这是right</p>
      <p>这是right</p>
      <p>这是right</p>
      <p>这是right</p>
      <p>这是right</p>
    </div>
  </div>
</body>
</html>